Of 408 negative axillary node (NAN) patients surgically treated at Roswell Park Cancer Institute (Buffalo, NY, 1976 through 1987), 36 (8.8%) presented with ductal carcinoma in situ with microinvasion (DCISM). In more than 50% of the patients (20/36) the disease was detected solely by mammography (microcalcifications and/or radiological density less than 1.5 cm). Thirty-three patients underwent modified radical mastectomy; three had wide excision ± axillary dissection. Residual disease after excisional biopsy was found in 22 of 33 mastectomy specimens (67%): 11 (33%) pure ductal carcinoma in situ, five (15%) DCISM, and six (18%) frankly invasive. Of the 22 incidents of residual disease, 50% (11) were multicentric (one third of all specimens). These findings imply a high likelihood of residual cancer after excisional biopsy in these patients. All patients were free of disease for a mean follow-up of 57 months (range, 16 to 137). These findings indicate that DCISM is an entirely curable disease when treated by mastectomy alone, without the need for adjuvant therapy, regardless of the status of other prognostic factors such as tumor size, histologic differentiation, age, or steroid receptor status.